In the Las Vegas area, we used to have many elementary schools (and some middle schools for a short time) that were on year-round calendars due to overcrowding. For those schools, school zone signage had a "12 Month" placard below the normal school zone sign assembly to notify drivers that the zones were in effect all year.
Aside from that, I don't think they did any special signage for regular 9-month schools where summer sessions may be held. Although I think Clark County School District only really did summer school for high school age students–aside from school zones adjacent to the high school (which typically use flashers instead of signs with static times), there aren't really school zones/crossing zones for high schools so there wouldn't be any signs to modify for summer.